1. Field of the Invention
This invention relates generally to an instrument using room-temperature sensors that measure magnetic susceptibility variations in the body of a patient. In particular, the instrument can noninvasively monitor ferromagnetic foreign bodies that may become lodged in a patient.
2. Background Art
There is a need for an accurate, noninvasive method to detect the presence of ferromagnetic foreign bodies in a patient who is being considered for magnetic resonance imaging.
As a matter of interest, biomagnetic susceptometry is a diagnostic procedure that involves noninvasive, radiation-free, direct, and accurate, measurement of the magnetic susceptibility of organs and tissue within a human or animal body. For instance, biomagnetic susceptometry can be used to measure human iron stores contained in the liver, see Harris, J. W., et al. (1978), Assessment of human iron stores by magnetic susceptibility measurements, Clin. Res. 26, 540A.; Brittenham, G. M., et al. (1993), Hepatic iron stores and plasma ferritin concentration in patients with sickle cell anemia and thalassemia major, Amer. J Hematology 42, 85; Brittenham, G. M., et al. (1982), Magnetic susceptibility of human iron stores, New England J Med, 307, 167 1.; Fischer, R., et al. (1992), Liver iron quantification in the diagnosis and therapy control of iron overload patients, Biomagnetism: Clinical. Aspects,:M. Hoke, et al., eds., Elsevier, Amsterdam, p. 585., 1992; Fischer, R., et al. (1989), in Advances In Biomagnetism, S. J. Williamson, et al., eds., Plenum, N.Y., p. 501. Paulson. D. N., et al. (1991), Biomagnetic susceptometer with SQUID instrumentation, IEEE Trans. Magnetics 27, 3249.; and Nielsen, P., et al. (1995), Liver iron stores in patients with secondary hemosideroses under iron chelation therapy with deferoxamine or deferiprone, Br. J Hematol. 91, 827.
Unfortunately, instruments based on Superconducting Quantum Interference Devices (SQUIDs), are complex and expensive. They also use liquid helium, leading to significant operating costs and supply problems. Only a few such devices are in use worldwide presently due to their complexity and expense.
SQUIDs based on the recently developed High-Temperature Superconductors (HTS) could, in principle, reduce the cost of magnetic suceptometry. HTS SQUIDs, which can operate at liquid-nitrogen temperatures, would reduce operating costs, and some of the equipment costs, compared to SQUID devices operating at liquid helium temperatures. However, even at liquid-nitrogen temperatures, the operating costs would be higher than those of ordinary instruments operating at room temperature. Moreover, HTS-SQUIDs are expensive to construct and use, because of the difficulty and low yield of the fabrication process. The difficulties, and the costs, are compounded because these devices are vulnerable to moisture, thermal cycling, and static electrical discharge. HTS-SQUIDs also require the same expensive electronics as conventional SQUIDs.
The instant invention obviates the need for cryogenically cooled SQUIDs by providing operational use at room temperature, making for much less expensive fabrication and use. The invention allows, generally for measurements of variations of magnetic susceptibility in a patient and, in particular an accurate and inexpensive way of detecting areas of increased magnetic susceptibility in patients. In addition, certain improvements introduced in this invention are applicable to all types of magnetic susceptibility measurements.
Broadly speaking, this invention provides a practical method and apparatus for measuring variations of magnetic susceptibilities in a patient, and, in particular, preferably localized areas of increased magnetic susceptibility. The probing instrument""s distal end assembly includes a room temperature functioning magnetic sensor that can detect the characteristic magnetic response from tissue to a magnetic field supplied by an applied-field coil that is also part of the instrument""s distal end assembly. The applied field coil can be an alternating current (AC) coil. The magnetic susceptibility measurements have sufficient resolution to monitor small variations in magnetic susceptibility within the patient, when the instrument is placed external to the patient.
The magnetic sensor can be, but is not necessarily limited to, a magnetoresistive sensor (including giant magnetoresistive and spin-dependent tunneling sensors), a fluxgate magnetometer, or a magneto-inductive sensor. The applied field coil dimensions are such that an applied field is optimized for maximum response from localized areas of interest in the body. In particular, the instrument of the present invention is preferably designed for detecting the presence of ferromagnetic foreign bodies (FFBs) in a patient. For this application, the applied field coil dimensions are optimized to maximize the magnetic susceptibility response from the item of interest and minimize effects caused by the overlying tissue, while not unduly increasing the sensitivity of the probe instrument due to an organ being in close proximity to the item of interest. To minimize noise introduced in the magnetic sensor due to fluctuations in the applied field, the applied field is canceled at the position of the sensor. Both the real and imaginary parts of the applied field are canceled. To overcome variations in the sensor output caused by changes in ambient temperature and mechanical relaxation of the instrument, the sensor-sample distance is modulated by oscillating the detector assembly. In contrast with conventional biomagnetic measurement instruments that use SQUID sensors, where a patient is moved relative to the instrument, the proposed invention""s magnetic sensor is moved relative to the patient. The instrument""s detector assembly has an applied field coil fabricated on a printed circuit (PC) board that is attached to a solid nonmetallic support base which in turn attaches to an oscillatory member which displaces the detector assembly when used for examining a patient.
The probe instrument""s distal end detector assembly has a geometrically designed applied field coil using either multiple parallel-sheet coils or a substantially coplanar applied field coil of concentric design. The magnetic sensor is preferably a magnetoresistive (MR) sensor. When an MR sensor is used, a feedback coil is mounted on the sensor, which xe2x80x9clocksxe2x80x9d the sensor at its optimum operating point by applying a compensating field to cancel changes in the ambient field, thus maintaining a constant sensitivity of the detector assembly.
The probing instrument""s magnetic sensor control electronics, a motor/crank rod arrangement for oscillatory movement of the instrument""s distal end detector assembly, an applied field source signal generator, a lock-in amplifier, an audio amplifier, and an FFT spectrum analyzer or equivalent computer device for signal analysis can all be incorporated in a single medical instrument housing for field use.
A physician uses the probing instrument by positioning the probe""s distal end adjacent to an area of interest, and the instrument""s detector assembly is preferably oscillated over the area of interest. The probe instrument then analyzes the observed signal, and outputs data corresponding to material of interest.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: